Very small pixel pitch focal plane array and method for manufacturing thereof

ABSTRACT

A method for manufacturing an imaging device is provided. The method comprises forming a contact pad over a semiconductor substrate. The contact pad has a malleable metal. The method further comprises providing a readout circuit having a first side and a contact plug. The contact plug has a base affixed to the first side of the readout circuit and a plurality of prongs extending from the base away from the first side. The first side of the readout circuit is moved towards the substrate so that the prongs of the contact plug are pressed into the pad and displace a portion of the pad into a space defined by and between a first and a second of the prongs. Stop elements formed over the substrate are aligned with and contact stop elements provided on the readout circuit so that the prongs are inhibited from passing completely through the contact pad.

FIELD OF THE INVENTION

The present invention relates to imaging devices. More particularly, thepresent invention relates to an imaging device having a focal planearray with a readout circuit and method of manufacturing thereof, whereeach pixel of the focal plane array has a very small pitch withdimensions corresponding to the cutoff wavelength of the photodetectorof the respective pixel.

BACKGROUND OF THE INVENTION

The image resolution that can be achieved from conventional infraredfocal plane arrays (IR FPA), even with the most favorable optics, aregenerally limited by the pixel pitch. In the most advanced conventionalFPAs, the smallest pixel pitch dimensions are 12 microns for midwavelength (MW) IR with a nominal cutoff wavelength of 5 microns. In thelong wavelength (LW) IR bands with cutoff wavelengths of around 10microns, the smallest pixel pitch observed by the applicant forconventional FPAs is 15 microns.

To enhance and further optimize image resolution, the pixel pitch of anFPA for an imaging device or photodetector needs to be comparable to thewavelength of radiation being detected. Among the primary limitations inreducing the pixel pitch of conventional imaging devices orphotodetectors are the architecture of the FPA and associated readoutcircuit contacts and the fabrication technique for electricalinterconnection of the FPA to the unit cells of the associated readoutcircuit. These limitations are especially compelling for the shorterwavelength IR bands with cutoff wavelengths of 2.5 microns or less.

Using high density vertically integrated photodiode (HDVIP®, a trademarkof DRS Technologies, Inc.) architecture, a pixel pitch as small as 6microns and via diameters as small as 2 microns are practicallyfeasible. With these dimensions, fill factors of approximately 90% maybe realized. FIG. 1 shows the variation in fill factor versus pixelpitch for an FPA implementing pixels with HDVIP® architecture and havingone of three different values of via diameters. As shown in FIG. 1, forsmaller pixel pitches (e.g., less than 6 microns), the fill factor dropsrapidly even for a via diameter of 2 microns and, thus, compromises theoverall photodetector performance.

There is therefore a need for an FPA with associated readout circuitcontact architectures and fabrication techniques that enables therealization of imaging devices with pixel pitches approaching thewavelength of radiation to be detected without compromising the fillfactor of each photodetector. This is especially a stressing requirementfor the short wavelength or SWIR spectral band with cutoff wavelengths≦2.5 microns. In addition to image resolution, smaller pixel pitch IRFPAs enable reduced size of optics, reduced cooling requirements, whichin turn leads to a smaller package, lower power consumption and reducedoverall weight.

SUMMARY OF THE INVENTION

In accordance with systems and articles of manufacture consistent withthe present invention, an imaging device having an improved focal planearchitecture is provided. The imaging device comprises a semiconductorlayer (such as an semiconductor infrared absorbing layer) and aphotodetector having an implanted region formed in the semiconductorlayer to define a p-n (or n-p) junction therein, and a pad formed ordeposited over the implanted region. The pad has a malleable metal ormetallic material, such as Indium. The imaging device further comprisesa readout circuit having a contact plug. The contact plug has a base anda prong extending from the base and into the malleable metallic materialof the pad. In one implementation, the prong is a first of a pluralityof prongs extending from the base and into the malleable metallicmaterial of the pad. The prongs have a structure effective to displace aportion of the malleable metallic material into a space between thefirst prong and a second of the prongs.

In addition, in accordance with methods consistent with the presentinvention, a method is provided for manufacturing an imaging device. Themethod comprises forming a contact pad having a malleable metallicmaterial over a surface of a semiconductor substrate (e.g., such the padis formed over the surface of a photodetector formed in thesemiconductor substrate), and providing a readout circuit having a firstside and a contact plug. The contact plug has a base affixed to thefirst side of the readout circuit and a plurality of prongs extendingfrom the base away from the first side. The method further comprisesmoving the first side of the readout circuit towards the substratesurface so that the prongs of the contact plug are pressed into the padand displace a portion of the pad into a space defined by and between afirst and a second of the prongs.

In one implementation, the method further comprises: forming a firstpair of stop elements over the semiconductor substrate surface so thatthe contact pad is disposed between the first pair of stop elements; andproviding a second pair of stop elements on the first side of thereadout circuit so that the base of each contact plug is disposedbetween the second pair of stop elements and in substantial alignmentwith the first pair of stop elements formed over the substrate surface.In this implementation, the first side of the readout circuit is movedtowards the substrate surface in substantial axial alignment with thefirst and second pairs of stop elements until the first pair of stopelements contacts the second pair of stop elements. A first of the firstpair of stop elements and a first of the second pair of stop elementshave a combined thickness that is more than a length of each prong suchthat the prongs are inhibited from passing completely through thecontact pad when the first pair of stop elements contacts the secondpair of stop elements.

In accordance with systems and articles of manufacture consistent withthe present invention, another imaging device having an improved focalplane architecture and effective to provide two color detection isprovided. The imaging device comprises a first semiconductor layerhaving a first surface and a second surface; and a first photodetectorhaving a first implanted region formed in the first semiconductor layerand a pad formed over the first implanted region. The pad has amalleable metallic material. The imaging device also comprises a readoutcircuit disposed over the first surface of the first semiconductorlayer. The readout circuit has a plurality of contact plugs facing thefirst surface of the first semiconductor layer. A first of the contactplugs has a first base and a first prong extending from the first baseand into the malleable metallic material of the pad of the firstphotodetector. The imaging device further comprises a secondsemiconductor layer disposed below the second surface of the firstsemiconductor and a second photodetector having a second implantedregion formed in the second semiconductor layer. In addition, theimaging device has a metalized via extending through the firstsemiconductor layer through an insulated via and the secondsemiconductor layer so that the metalized via electrically only connectsthe second implanted region of the second photodetector to a second ofthe contact plugs of the readout circuit, enabling the imaging device todetect two wavelength bands or two portions of a band (e.g., two colorsof the visible band or infrared band).

Other systems, methods, features, and advantages of the presentinvention will be or will become apparent to one with skill in the artupon examination of the following figures and detailed description. Itis intended that all such additional systems, methods, features, andadvantages be included within this description, be within the scope ofthe invention, and be protected by the accompanying claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate an implementation of the presentinvention and, together with the description, serve to explain theadvantages and principles of the invention. In the drawings:

FIG. 1 is a graph depicting the variation in the fill factor percentageof a conventional HDVIP® photodetector of a pixel as a function of thepixel's pitch for three different “via” diameters for various pixelpitch on a focal plane array (FPA);

FIGS. 2A-2B show a flow chart depicting a process for manufacturing animaging device in which a photodetector array and a ROIC are aligned andinterconnected in accordance with the present invention;

FIGS. 3A to 3F and 3H to 3J are cross sectional views of an exemplaryphotodetector array and an exemplary ROIC of an imaging devicemanufactured in accordance with the process depicted in FIG. 2, wherethe photodetector array and the ROIC are illustrated at various steps ofthe manufacturing process;

FIG. 3G is a top level view of the exemplary photodetector arraycorresponding to the cross-sectional view in FIG. 3F and before the ROIChaving one or more contact plugs is applied in accordance with thepresent invention to the photodetector array as depicted in FIGS. 3H-3J;and

FIG. 4 is a cross sectional view of another imaging device manufacturedin accordance with the present invention, in which the imaging devicehas an exemplary ROIC, a first exemplary photodetector arraymanufactured in accordance with the process depicted in FIG. 2, and asecond exemplary photodetector array that collectively form a two colorfocal plane array for the imaging device.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to an implementation in accordancewith methods, systems, and products consistent with the presentinvention as illustrated in the accompanying drawings.

Methods consistent with the present invention provide a process 200depicted in FIGS. 2A-2B for manufacturing a focal plane array (FPA) ofan imaging device, where an array of photodetectors with dimensionscomparable to the cutoff wavelength of the respective photodetector isaligned with and interconnected to a unit cell of a readout integratedcircuit (ROIC or readout circuit) to enable the realization of verysmall pixel pitches. FIGS. 3A to 3F and 3H to 3J are cross sectionalviews of an exemplary photodetector array 302 and an exemplary ROIC 304of an imaging device 300 (as completed in FIG. 3J), where thephotodetector array 302 and the ROIC 304 are illustrated at varioussteps of the manufacturing process 200. FIG. 3G is a top level view ofthe exemplary photodetector array 302 corresponding to thecross-sectional view in FIG. 3F and before the ROIC 304 having one ormore contact plugs 350 a-350 c is applied in accordance with the presentinvention to the photodetector array 302 as depicted in FIGS. 3H-3J.Each contact plug 350 a-350 c is associated with and reflects arespective unit cell of the ROIC 304.

As shown in FIG. 2A and FIG. 3A, a passivation layer 310 is initiallyformed over a semiconductor substrate or layer 312 having a firstconductivity type (step 202). In a pre-processing step, thesemiconductor layer 312 may initially be formed over or deposited on asubstrate (not shown in figures) comprising cadmium zinc telluride(e.g., when the semiconductor layer 312 comprises mercury-cadmiumtelluride), indium phosphide (e.g., when the semiconductor layer 312comprises indium gallium arsenide), or other material suitable forforming a semiconductor layer. The substrate upon which thesemiconductor layer 312 is formed may be removed using any knownsemiconductor device manufacturing technique, which is not described toavoid obscuring the present invention.

The passivation layer 310 may comprise cadmium telluride (CdTe), cadmiumzinc telluride, cadmium telluride selenium, zinc sulfide, or any othersuitable passivation material. The passivation layer 310 has a thicknessin a range of 30 nm to 250 nm.

In one implementation, the semiconductor substrate or layer 312comprises an infrared sensitive material, such as mercury cadmiumtelluride (HgCdTe), mercury zinc telluride (HgZnTe), mercury cadmiumzinc telluride (HgCdZnTe), cadmium telluride (CdTe), cadmium zinctelluride (CdZnTe), indium gallium arsenide (InGaAs), or indiumantimonide (InSb), doped with a material, such as Arsenic (As) or Copper(Cu), to form a p-type semiconductor layer. Alternatively, thesemiconductor layer 312 may comprise silicon, germanium, galliumarsenide (GaAs), indium antimonide (InSb), other III V or II VI compoundsemiconductors, or the like, suitable for forming a photodetector. In analternative implementation, the semiconductor substrate or layer 312 maybe doped with another material so that the semiconductor layer 312 has asecond conductivity type (e.g., n-type) different from the firstconductivity type (e.g., p-type). For example, the semiconductorsubstrate or layer 312 may be doped with Boron (B) or other n-typematerial layer. Accordingly, each photodetector (as reflected byimplanted regions 314 a-314 c in FIG. 3B) fabricated in thephotodetector array 302 in accordance with the present invention mayhave a p-on-n or a n-on-p architecture and corresponding junctionwithout departing from the scope of the present invention.

In addition, as shown in FIGS. 3A-3F and 3H-3J, the starting detectormaterial may have another passivation layer 316 formed on a back-side318 of the semiconductor layer 312 to form a double sided passivatedsemiconductor layer 302, allowing for front-side illumination of thephotodetector array 302. In an alternative implementation, themanufacturing steps discussed here may be applied so that thephotodetectors of the array 302 are formed on the back-side 318 of thesemiconductor layer 302 with the passivation layer 310 remaining intactover the front-side of the semiconductor layer 302, allowing forback-side illumination of the photodetector array 302.

Continuing with FIG. 2A, the passivation layer 310 is patterned usingphotolithography and etched using a dry or a wet etching technique, toform openings such as 320 a, 320 b and 320 c in the passivation layer310 (step 204) that expose the substrate 312. One of ordinary skill inthe art would appreciate that other patterning and etching techniquesmay be employed without departing from the present invention. Theopenings 320 a, 320 b and 320 c may each have a width (w) within therange of 0.3 μm to 1 μm (and preferably less than 0.8 μm), allowing eachphotodetector (as represented by the implanted regions 314 a-314 c) tobe formed to a pixel pitch that is approximately equal to a cutoffwavelength of the radiation to be detected by the respectivephotodetector.

Next, one or more implanted regions 314 a, 314 b and 314 c are formed inthe substrate, via the one or more openings, where each implanted regionhas a second conductivity type that is different than the firstconductivity type of the substrate 312 (step 206). For example, when thesemiconductor layer 312 is comprised of HgCdTe doped with Arsenic (As)to have a p-type conductivity, Boron (B+) may be implanted, via eachopening 320 a, 320 b and 320 c, into the substrate 312 to form implantedregions 314 a, 314 b or 314 c having an n-type conductivity. Eachimplanted region 314 a, 314 b or 314 c having the second conductivitytype forms a junction with the substrate 312 having the firstconductivity type to effectively form a n-on-p architecture associatedwith a respective photodetector. Alternatively, each photodetector maybe implemented to have a p-on-n junction architecture when thesemiconductor layer 312 is doped with Indium, for example, to haven-type conductivity. Note, in an alternative implementation, followingthe patterning, the implants may be formed through the passivation layerbefore the formation of the contact openings 320 a, 320 b and 320 c viaetching.

Each implanted region 314 a, 314 b and 314 c has the approximate width(w) of the opening 320 a, 320 b or 320 c through which the respectiveimplanted region was formed. Thus, the photodetector represented by therespective implanted region 314 a, 314 b or 314 c is formed to a pixelpitch that is approximately equal to a cutoff wavelength of theradiation to be detected by the respective photodetector. For example,the pixel pitch (e.g., d₁, as shown in FIG. 3H) of the photodetectorrepresented by implanted region 314 a may be approximately 2.5 micronsor less, corresponding to the cutoff wavelength of the SWIR spectralband.

After forming the implanted regions 314 a, 314 b and 314 c, a basecontact 322 a, 322 b or 322 c is formed over each implanted region 314a, 314 b and 314 c in each contact opening 320 a, 320 b and 320 c (step208). Each base contact 322 a, 322 b or 322 c is comprised of a metal ormetal alloy and is formed to a thickness substantially equal to thethickness of the initial passivation layer 310. In one implementation,each base contact 322 a, 322 b and 322 c has two layers 324 a and 326 a,324 b and 326 b or 324 c and 324 c. The first layer 324 a, 324 b or 324c is comprised of a first type of material that substantially bonds oradheres to the material comprising the implanted region 314 a, 314 b or314 c (e.g., Boron doped HgCdTe). The second layer 326 a, 326 b or 326 cis comprised of a second type of material that substantially bonds oradheres to the first layer material and to the malleable metallicmaterial comprising each pad (e.g., each pad 332 a, 332 b or 332 c inFIG. 3F comprises Indium). In one implementation, the first layer 324 a,324 b or 324 c of each base contact comprises Nickel (Ni) deposited overa respective implanted region 314 a, 314 b or 314 c to have a thicknessof approximately 100 Angstroms or within a range of 50 Angstroms to 150Angstroms. In this implementation, the second layer 326 a, 326 b or 326c of each base contact comprises Titanium (Ti) deposited over arespective first layer 324 a, 324 b or 324 c to have a thickness ofapproximately 100 Angstroms or within a range of 50 Angstroms to 1000Angstroms. The thickness of the Titanium is equal to or greater than thethickness of the Nickel in the base contact. In one implementation, thethickness of the Nickel is sufficient to enable the Titanium portion ofthe base contact to adhere to a respective pad 332 a, 332 b or 332 ccomprised of Indium. In this implementation, the second layer 326 a, 326b or 326 c of Titanium adheres better than Nickel to the Indium used toform the contact pad.

As shown in FIG. 2A and FIG. 3D, a self-limiting photo-resist layer 328is then deposited or formed over each base contact 322 a, 322 b or 322 cand remaining initial passivation layer 310 to have a first thicknesswithin a range of 0.5 μm to 1 μm (step 210). As shown in FIG. 3E, a padopening 330 a, 330 b or 330 c is formed (e.g., via patterning andetching) in the photo-resist layer 328 over each base contact 322 a, 322b and 322 c (step 212). Each opening 330 a, 330 b and 330 c may extendfrom the top of the photo-resist layer 328 to the base contact 322 a,322 b and 322 c, which may be exposed via the etching process.

Next, a pad 332 a, 332 b or 332 c comprised of a malleable metallicmaterial or alloy, such as Indium or suitable Indium alloy, is formed ineach pad opening 330 a, 330 b and 330 c (step 214) as shown in FIG. 3F.In one implementation, each pad 332 a, 332 b and 332 c is formed to havethe first thickness of the photo-resist layer 328.

As depicted in FIG. 3F, the photo-resist layer 328 is then patterned andetched to form at least a first pair of stop elements (e.g., 334 a and334 d) on the initial passivation layer 310 so that each pad 332 a, 332b and 332 c is disposed between the first pair of stop elements (e.g.,334 a and 334 d) and each of the first pair of stop elements has asecond thickness that is different than the first thickness of the pad(step 216). As discussed in further detail below, at least the firstpair of stop elements (e.g., 334 a and 334 d) on the photodetector array302 are adapted to contact at least a second pair of stop elements(e.g., 362 a and 362 d in FIG. 3H) on the readout circuit 304 so as toinhibit further movement of the readout circuit 304 towards thephotodetector array 302, preventing damage to the base contacts 322 a,322 b and 322 c and underlying photodetector implanted regions 314 a,314 b and 314 c. In the implementation shown in FIGS. 3F and 3G, thefirst pair of stop elements may be two of a plurality of stop elements334 a-334 l formed from the photo-resist layer 328 on the initialpassivation layer 310 of the photodetector array 302 in accordance withthe present invention. In one implementation, each pad 332 a, 332 b, 332c, 332 d, 332 e and 332 f may be disposed between a respective pair ofstop elements. For example, as shown in FIG. 3G, a pad 332 a may bedisposed between a pair of stop elements 334 a and 334 b located nearopposing sides 336 and 338 of the pad 332 a or between a pair of stopelements 334 a and 334 f located near opposing corners 334 a and 334 fso as to contact a corresponding pair of stop elements on the readoutcircuit 304 so as to inhibit further movement of the readout circuit 304towards the respective pad 332 a of the photodetector array 302,preventing damage to the base contact 322 a and photodetector implantedregion 314 a underlying the pad 332 a. To provide further reliability inconnecting the ROIC 304 to the photodetector array 302 as furtherdescribed herein, each pad 332 a-332 f may be disposed between arespective four stop elements located about the pad 332-333 f. Forexample, as shown in FIG. 3G, each pad (e.g., pad 332 a) may be disposedbetween a respective four stop elements (e.g., 334 a, 334 b, 334 e and334 f) located near corners (e.g., 340, 342, 344 and 346) of therespective pad.

In addition, although the pads 332 a-332 f are shown in FIG. 3G ashaving a square shape, each pad 332 a-332 f may have a polygon, circularor other shape without departing from the spirit of the presentinvention. Furthermore, although the stop elements 334 a-334 l aredepicted as square posts, the stop elements 334 a-334 l may also have apolygon, circular or other shape and be formed in a strip or linewithout departing from the spirit of the present invention. Moreover, inan alternative implementation, each pad 332 a-332 f may be surrounded onat least three sides by a single stop element (e.g., 334 a) formed in astrip or line.

Turning to FIG. 2B, a readout circuit 304 is formed or provided that hasa first side 348 and a contact plug 350 a, 350 b or 350 c for each pad332 a, 332 b and 332 c (step 218). Each contact plug 350 a, 350 b or 350c has a base 352 a, 352 b and 352 c affixed to the first side 348 of thereadout circuit 304 and one or a plurality of prongs 354 a-354 c, 356a-356 c or 358 a-358 c, extending from the base 352 a, 352 b or 352 caway from the first side 348 of the ROIC 304. Each contact plug 350 a,350 b or 350 c may comprise a material (such as Tungsten) that may bepressed into and substantially bond or adhere to the malleable metallicmaterial or metal alloy (e.g., Indium or alloy thereof) comprising arespective pad 332 a-332 f, either with or without an annealingprocessing step. Each prong of a respective contact plug 350 a, 350 b or350 c may have a polygon, circular or other shape. Each prong 354 a-354c, 356 a-356 c or 358 a-358 c may extend a length (L) from the top side348 of the ROIC 304 that is within a range of 0.25 microns to 0.5microns. In addition, each prong 354 a-354 c, 356 a-356 c or 358 a-358 cmay have a diameter or width (w) is within a range of 0.25 microns to0.5 microns. Each set of adjacent prongs (e.g., 354 a and 354 b) definea space 360 therebetween. In one implementation, the combined width ofthe prongs and spaces 360 between adjacent prongs for a contact plug(e.g., 350 a) does not exceed the width (d) of the pad (e.g., 332 a) towhich the contact plug is to be connected as described below or thepixel pitch (d₁) of the photodetector associated with the pad (e.g., 332a).

As shown in FIG. 2B, a second pair of stop elements (e.g., stop elements362 a and 362 d in FIG. 3H) are provided or formed on the first side 348of the readout circuit 304 so that the base 352 a, 352 b or 352 c ofeach contact plug 350 a, 350 b and 350 c is disposed between the secondpair of stop elements (e.g., stop elements 262 a and 262 d) and insubstantial axial alignment with the first pair of stop elements (e.g.,334 a and 334 d) of the photodetector array 302 (step 220). Each of thestop elements of each contact plug 350 a, 350 b and 350 c has a thirdthickness such that the combined thickness of a stop element (e.g., 362a) of a contact plug (e.g., 350 a) and a corresponding stop element(e.g., 334 a) of the photodetector array 302 is more than the length (L)of each prong (e.g., 354 a-354 c) of the respective contact plug (e.g.,350 a) such that the prongs (e.g., 354 a-354 c) are inhibited frompassing completely through the associated pad (e.g., 332 a).

In the implementation shown in FIG. 3H, the second pair of stop elementsof the readout circuit 304 may be two of a plurality of stop elements362 a-362 d provided or formed on the first side 348 of the readoutcircuit 304. In one implementation, each contact plug 350 a, 350 b and350 c may be disposed between a respective pair of stop elements 362a-362 d. For example, as shown in FIG. 3H, a contact plug 350 a may bedisposed between a pair of stop elements 362 a and 362 b located nearopposing sides (as reflected by prongs 354 a and 354 c) of the contactplug 350 a, or between a pair of stop elements located near opposingends or corners (not shown in figures) of the contact plug 350 a. In theimplementation of the photodetector array 302 shown in FIG. 3G, thereadout circuit 304 may have a stop element 362 a-362 l (elements 362e-362 l not shown in the figures) for each stop element 334 a-334 l ofthe photodetector array 302. In this implementation, each of the stopelements 362 a-362 l of the readout circuit 304 are substantiallyaligned with and adapted to contact a corresponding stop element (e.g.,element 334 a-334 l) of the photodetector array 302 so as to inhibitfurther movement of the readout circuit 304 towards the pads 332 a-332 fof the photodetector array 302, preventing damage to the base contactand photodetector implanted region underlying each pad 332 a-332 f.

Continuing with FIG. 2B, the first side 348 of the readout circuit 304is moved towards the semiconductor substrate or layer 312 of thephotodetector array 302 in substantial axial alignment with the stopelements 334 a-334 d of the photodetector array and the stop elements362 a-362 d of the readout circuit 304 such that the prongs 354 a-354 c,356 a-356 c or 358 a-358 c of each contact plug 350 a, 350 b and 350 care pressed into each pad 332 a, 332 b and 332 c and displace a portion(e.g., 364 a, 364 b, 364 c, 364 d, 364 e or 364 f) of the respective padinto the space 360 between adjacent prongs (step 222).

The first side 348 of the readout circuit 304 continues to be movedtowards the photodetector array 302 in substantial axial alignment withthe stop elements 334 a-334 d of the photodetector array and the stopelements 362 a-362 d of the readout circuit 304 until each stop elementof the photodetector array 302 (or the first pair of stop elements 334 aand 334 d) contacts a corresponding stop element of the readout circuit304 (or the second pair of stop elements 362 a and 362 d) as shown inFIG. 3I. As previously noted, the combined thickness of each stopelement of the photodetector array 302 (e.g., each of the first pair ofstop elements 344 a and 334 d) and the corresponding stop element of thereadout circuit 304 (e.g., corresponding one of the second pair of stopelements 362 a and 362 d) is more than the length (L) of each prong 354a-354 c, 356 a-356 c and 358 a-358 c such that the prongs are inhibitedfrom passing completely through the respective pad 332 a, 332 b or 332 c(step 224).

In an alternative implementation, step 220 is skipped and stop elements362 a, 362 b, 362 c and 362 d are not formed that the first side 348 ofthe readout circuit 304. In this implementation, in step 222, the firstside 348 of the readout circuit 304 is moved towards the semiconductorsubstrate or layer 312 of the photodetector array 302 such that theprongs 354 a-354 c, 356 a-356 c or 358 a-358 c of each contact plug 350a, 350 b and 350 c are pressed into a respective one of the pads 332 a,332 b and 332 c and displace a portion (e.g., 364 a, 364 b, 364 c, 364d, 364 e or 364 f) of the respective pad into the space 360 betweenadjacent prongs. In addition, in this implementation in step 224, thefirst side 348 of the readout circuit 304 continues to be moved towardsthe photodetector array 302 until each stop element of the photodetectorarray 302 (or the first pair of stop elements 334 a and 334 d) contactsthe first side 348 of the readout circuit 304. In this implementation,the thickness of each stop element of the photodetector array 302 (e.g.,each of the first pair of stop elements 344 a and 334 d) is more thanthe length (L) of each prong 354 a-354 c, 356 a-356 c and 358 a-358 csuch that the prongs are inhibited from passing completely through therespective pad 332 a, 332 b or 332 c.

In one implementation, to facilitate the hybridization of each contactplug 350 a-350 c of the readout circuit 304 to a corresponding pad 344a-344 c of the photodetector array 302, the photodetector array 302 maybe warmed to a predetermined temperature that is below the melting pointof the material used to form each pad 332 a-332 c. For example, wheneach pad 332 a-332 c is comprised of Indium, the photodetector array 302may be warmed to a predetermined temperature that is equal to or lessthan 80° C.

Before or while the photodetector array 302 is being warmed, epoxy 366(shown in FIG. 3J) may be injected or wicked into a cavity 368 a-368 f(shown in FIG. 3I), a portion of which is defined by two or more of thefollowing: a pad 332 a, 332 b or 332 c; a stop element 334 a-334 n ofthe photodetector array 302 (e.g., a first 334 a of the first pair ofstop elements 334 a and 334 d); a corresponding stop element 362 a-362 nof the readout circuit 304 (e.g., a first 362 a of the second pair ofstop elements 362 a and 362 d) in contact with the stop element (e.g.,334 a) of the photodetector array 302; the initial passivation layer 310and the first side 348 of the readout circuit 304 (step 226). Note thepredetermined temperature may be above the melting point of the materialused to form the photo-resist layer 328 from which the stop elements 344a-344 d of the photodetector array 302 are formed. In thisimplementation, the stop elements 344 a-334 d and/or the epoxy 366 flowsto surround and encapsulate the perimeter of each pad 332 a, 332 b or332 c, preventing excess malleable metallic material (e.g., Indium) ofone pad (e.g., pad 332 a) from shorting to an adjacent pad (e.g., 332 bor 332 d in FIG. 3G). Once the photodetector array 302 is cooled back toroom temperature, the wicking process is ended as the epoxy 366 and/orstop elements 344 a-334 d made from photo-resist material solidify. Thecombination of the photodetector array 302 and the respective unit cellsof the ROIC 304 form the FPA of the imaging device 300, and may bemounted on a chip carrier.

Turning to FIG. 4, another imaging device 400 manufactured consistentwith the present invention is shown. The imaging device 400 incorporatesthe ROIC 304 (i.e., ROIC 404 in FIG. 4) and the photodetector array 302(i.e., photodetector array 402 in FIG. 4) of the imaging device 300. Thephotodetector array 402 and ROIC 404 are each manufactured and connectedto each other in accordance with the process depicted in FIG. 2 aspreviously discussed, except as noted below. As shown in FIG. 4, theimaging device 400 also includes a second photodetector array 406 formedbelow the first photodetector 402. As further described herein, the ROIC404, the first photodetector array 402 and the second photodetectorarray 406 collectively form a two color focal plane array of the imagingdevice 400, in which each unit cell (as shown in FIG. 4) of the focalplane array has a smaller pitch (e.g., D equal to or less than 15 μm)than other conventional two color imaging devices.

Consistent with the photodetector array 302 and the manufacturingprocess depicted in FIG. 2, the photodetector array 402 of the imagingdevice 400 includes a first semiconductor layer 312 having a firstsurface 317 (which may be a front-side surface) and a second surface 318(which may be a back-side surface) upon which a respective passivationlayer 310 or 316 is formed. The first semiconductor layer 312 has afirst conductivity type (e.g., p-type) and include an infrared sensitivematerial, such as HgCdTe, HgZnTe, HgCdZnTe, CdTe, CdZnTe, InGaAs orInSb. A first implanted region 314 a is formed in the firstsemiconductor layer 312 to form a p-on-n or a n-on-p architecture for afirst photodetector of the first photodetector array 402.

A base contact 322 a is formed over the implanted region 314 a. The basecontact 322 a is comprised of a metal or metal alloy and is formed to athickness substantially equal to the thickness of the initialpassivation layer 310. As previously described, the base contact 322 amay have two layers 324 a and 326 a. In this implementation, the firstlayer 324 a is comprised of a first type of material (e.g., Nickel) thatsubstantially bonds or adheres to the material comprising the implantedregion 314 a (e.g., Boron doped HgCdTe). The second layer 326 a iscomprised of a second type of material (e.g., Titanium) thatsubstantially bonds or adheres to the first layer material and to themalleable metallic material comprising the pad 332 a, which is formed onthe base contact 332 a for the first photodetector (as reflected by theimplant 314 a) of the photodetector array 402. In one implementation,the malleable metallic material comprising the pad 332 a is Indium or asuitable Indium alloy.

The ROIC 404 is disposed over the first surface 317 of the firstsemiconductor layer 312 and the passivation layer 310 formed thereon.The ROIC 404 has a plurality of contact plugs (e.g., 350 a and 450 a inFIG. 4) facing the first surface 317 of the first semiconductor layer312. A first 350 a of the contact plugs 350 a and 450 a has a first base352 a and one or more prongs 354 a-354 c extending from the first base350 a and into the malleable metallic material of the pad 332 a of thefirst photodetector. A second 450 a of the contact plugs is disposedadjacent to the first contact plug 350 a. The second contact plug 450 aincludes a second base 452 a and may include one or more prongs 454 aand 454 b extending from the second base 452 a.

The second photodetector array 406 includes a second semiconductor layer412 disposed below the second surface 318 of the first semiconductorlayer 312. A respective passivation layer 410 and 416 may be formed on afront-side surface 417 and back-side surface 418 of the secondsemiconductor layer 412 in the same manner as described for the firstsemiconductor layer 312.

A second implanted region 414 is formed in the second semiconductorlayer 412 adjacent to and below (but not directly beneath) the firstimplant region 314 a of the first semiconductor layer 312. The secondsemiconductor layer 412 has a conductivity type (e.g., p-type) that isdifferent from the conductivity type (e.g., n-type) of the secondimplanted region 414 to form the p-on-n or n-on-p architecture for thesecond photodetector in the second semiconductor layer 412.

In one implementation as shown in FIG. 4, two or more layers 419 a-419 bof filler material and/or epoxy may be disposed between and used toattach the lower passivation layer 316 formed on the second surface 318of the first semiconductor layer 312 and the upper passivation layer 410formed on the front-side 417 or upper surface of the secondsemiconductor layer 412.

As shown in FIG. 4, the imaging device 400 includes a metalized via 420extending through the first photodetector array 402 (and the firstsemiconductor layer 312 thereof) and through the second photodetectorarray 406 (and the second semiconductor layer 412 thereof) so that themetalized via 420 electrically connects the second implanted region 414formed in the second semiconductor layer 412 to the second base 452 a orprong 454 a or 454 b of the second contact plug 450 a of the ROIC 404.Thus, each unit cell of the ROIC 404 has two contact plugs 350 a and 450b, each of which is connected to a respective photodetector (asreflected by implants 314 a and 414) formed in one of the twosemiconductor layers 312 and 412, enabling the imaging device 400 todetect two different wavelengths or colors in a predetermined band.

The metalized via 420 may be formed using known via boring techniques.In one implementation, the second implant region 414 in the secondsemiconductor layer 412 (as well as a third implant region 422 in thefirst semiconductor layer 312) is formed during the via boring process.In one process for forming the metalized via 420, the firstphotodetector array 402 and the ROIC 404 are first formed and connectedtogether in accordance with the manufacturing process depicted in FIG.2. A first bore hole (having side walls 426 defined by the third implantregion 422 in FIG. 4) is then formed through the first semiconductorlayer 312 (and the passivation layers 310 and 316 sandwiching the firstsemiconductor layer 312) in perpendicular alignment with the secondcontact plug 450 a of the ROIC 404. An insulation film 424 is thendeposited on the side walls 426 of the first bore hole using knowndeposition techniques to prevent contact between the metalized via 420and the first semiconductor layer 312 or the third implant region 422therein. In one implementation, the insulation film 424 may be depositedso that the insulation film 424 extends through the first semiconductorlayer 312 to the base 452 a of the second contact plug 450 a. Ifnecessary, the first bore hole may be re-bored at a smaller diameter inorder to remove excess insulation or insulation blocking access to thebase 452 a of the second contact plug 450 a. A first portion 428 of themetalized via 420 may then be deposited over the insulation film 424 inthe first bore hole so that the metalized via 420 is electricallyconnected to the second contact plug 450 a but not the firstsemiconductor layer 312 or the third implant region 422 that may beformed therein during the boring process. A filler material 430, such asepoxy, may be deposited over the first portion 428 of the metalized via420 to fill any excess area in the first bore hole. A second portion 432of the metalized via 420 may be deposited as a layer on top of thefiller material 430 to provide a base contact extension in proximity tothe second or back-side surface 318 of the first semiconductor layer312.

Next, a second bore hole (having side walls 434 defined by the secondimplant region 414) is formed through the second semiconductor layer 412(and the passivation layers 410 and 416 sandwiching the secondsemiconductor layer 412) in perpendicular alignment with base contactextension (e.g., the second portion 432 of the metalized via 420) andthe second contact plug 450 a of the ROIC 404. A third portion 436 ofthe metalized via 420 may then be deposited on the side walls 434 of thesecond bore hole so that the metalized via 420 electrically connects thesecond implant region 414 to the base contact extension 432 and, thus,to the second contact plug 450 a. Thus, the ROIC 404 is structured toread the first photodetector defined by the first implant region 314 ain the first semiconductor layer 312 and to read the secondphotodetector defined by the second implant region 414 in the secondsemiconductor layer 412, where the second implant region 414 is disposedadjacent to and below (but not directly beneath) the first implantregion 314 a of the first photodetector in the first semiconductor layer312. In this implementation, the first photodetector as defined by thefirst implant region 314 a is effective to detect a first wavelengthassociated with a first portion of a predetermined band (e.g., thevisible band or infrared band) that passes through the secondsemiconductor layer and into the first semiconductor layer 312. Thesecond photodetector defined by the second implant region in the secondsemiconductor layer is effective to detect a second wavelengthassociated with a second portion of the predetermined band. Wavelengths460 associated with the first portion of the predetermined band anddetected by the first photodetector in the first semiconductor layer 312are longer than wavelengths 470 associated with the second portion ofthe predetermined band and detected by the second photodetector in thesecond semiconductor layer 412.

In accordance with the present invention, the imaging device 400 furthercomprises a first pair of stop elements (e.g., 334 a and 334 b) each ofwhich is disposed over the first surface 317 of the first semiconductorlayer 312 such that the pad 332 a of the first photodetector is disposedbetween the first pair of stop elements 334 a and 334 b. The ROIC 404has a second pair of stop elements (e.g., 362 a and 362 b) disposed onthe first side 348 of the ROIC 404. The base 352 a of the first contactplug 350 a is affixed to the first side 348 of the ROIC 404 such thatthe prong 354 a of the first contact plug 350 a extends away from thefirst side 348 of the ROIC 404 and between the second pair of stopelements 362 a and 362 b. When the ROIC 404 is moved towards the firstsurface 317 of the first semiconductor layer 312, the first pair of stopelements 334 a and 334 b formed over the semiconductor layer 312 (andformed on the passivation layer 310 in one implementation) contact thesecond pair of stop elements 362 a and 362 b of the ROIC 404 such thateach prong 354 a-354 c of the first contact plug 322 a is inhibited frompassing completely through the pad 332 a of the first photodetector.

As shown in FIG. 4, the imaging device 400 further comprises a thirdpair of stop elements 480 a and 480 b each of which is disposed over thefirst surface 317 of the first semiconductor layer 312 such that themetalized via 420 is disposed between the third pair of stop elements480 a and 480 b. The ROIC 404 has a fourth pair of stop elements 490 aand 490 b disposed on the first side 484 of the ROIC 404. The third pairof stop elements 480 a and 480 b is disposed relative to and contactingthe fourth pair of stop elements 490 a and 490 b such that each prong454 a and 454 b of the second contact plug 450 a is inhibited fromcontacting the first surface 318 of the first semiconductor layer 312.

As previously discussed, when the first side 348 of the ROIC 404 ismoved towards the first semiconductor layer 312 of the firstphotodetector array 402 in substantial axial alignment with the stopelements 334 a-334 b and 480 a-480 b of the first photodetector arrayand the stop elements 362 a-362 b and 490 a-490 b of the ROIC 404, theprongs 354 a-354 c of the first contact plug 350 a is pressed into thepad 332 a and displaces a portion of the pad 332 a into the spacebetween adjacent prongs (e.g., 354 a and 354 b or 354 b and 354 c).

While various embodiments of the present invention have been described,it will be apparent to those of skill in the art that many moreembodiments and implementations are possible that are within the scopeof this invention. Accordingly, the present invention is not to berestricted except in light of the attached claims and their equivalents.

What is claimed is:
 1. An imaging device, comprising: a semiconductorlayer; a photo detector having: an ion implanted region formed in thesemiconductor layer, the ion implanted region having a lateral spatialextent; and a pad spatially overlapping with the lateral spatial extentof the ion implanted region, the pad having a malleable metallicmaterial; and a readout circuit having a contact plug, the contact plughaving a base and a prong extending from the base and into the pad,wherein the photo detector is characterized by a pixel pitch that isapproximately equal to a cutoff wavelength corresponding to a shortwavelength infrared (SWIR) spectral band.
 2. An imaging device accordingto claim 1, wherein the malleable metallic material is Indium.
 3. Animaging device according to claim 1, wherein the prong comprises aconductive material adapted to substantially bond to the malleablemetallic material of the pad.
 4. An imaging device according to claim 3,wherein the conductive material is Tungsten.
 5. An imaging deviceaccording to claim 1, wherein the prong is a first of a plurality ofprongs extending from the base and into the pad, the prongs beingeffective to form electrical contact with and displace a portion of themalleable metallic material into a space between the first prong and asecond of the prongs.
 6. An imaging device according to claim 1, whereinthe imaging device further comprises a first pair of stop elements eachof which is disposed over the semiconductor layer such that the pad isdisposed between the first pair of stop elements, and wherein thereadout circuit has a first side and a second pair of stop elementsdisposed on the first side, the base of the contact plug being affixedto the first side such that the prong extends away from the first sideand between the second pair of stop elements, the first pair of stopelements being disposed relative to and contacting the second pair ofstop elements such that the prong of the contact plug is inhibited frompassing completely through the pad.
 7. An imaging device according toclaim 6, wherein the prong is a first of a plurality of prongs extendingfrom the base, away from the first side of the readout circuit and intothe pad, a first prong and a second of the prongs defining a spacetherebetween and being effective to collectively displace a portion ofthe malleable metallic material into the space between the first prongand the second prong.
 8. An imaging device according to claim 1, whereinthe imaging device further comprises a first pair of stop elements eachof which is disposed over the semiconductor layer such that the pad isdisposed between the first pair of stop elements, and wherein thereadout circuit has a first side, the base of the contact plug beingaffixed to the first side of the readout circuit such that the prongextends away from the first side and between the first pair of stopelements, the first pair of stop elements being disposed relative to andcontacting the first side of the readout circuit such that the prong ofthe contact plug is inhibited from passing completely through the pad.9. An imaging device according to claim 8, wherein the prong is a firstof a plurality of prongs extending from the base of the contact plug,away from the first side of the readout circuit and into the pad, afirst prong and a second of the prongs defining a space therebetween andbeing effective to collectively displace a portion of the malleablemetallic material into the space between the first prong and the secondprong.
 10. An imaging device according to claim 1, wherein the pad has awidth that is within the range of 0.3 μm to 8 μm.
 11. An imaging deviceaccording to claim 10, wherein the prong is a first of a plurality ofprongs extending from the base, away from the first side of the readoutcircuit and into the pad, each adjacent pair of the prongs defining aspace therebetween and being effective to collectively displace aportion of the malleable metallic material into the space between therespective adjacent pair of prongs, each of the prongs and each spacebetween each adjacent pair of prongs have a combined width equal to orless than the width of the pad.
 12. An imaging device according to claim1, wherein each prong has a width that is within the range of 0.25 μm to0.5 μm.
 13. An imaging device according to claim 1, wherein the pixelpitch is approximately 2.5 microns or less.
 14. An imaging deviceaccording to claim 1, wherein the pad is disposed between the ionimplanted region and the contact plug.
 15. An imaging device accordingto claim 1, further comprising at least one adhesion layer bonded to theion implanted region.
 16. An imaging device according to claim 15,wherein the at least one adhesion layer comprises nickel.
 17. An imagingdevice according to claim 15, further comprising a second adhesion layerbonded to the at least one adhesion layer, wherein the second adhesionlayer comprises titanium and is bonded to the pad.